The objective of this project is to understand the origin of the catalytic power of bacterial iron superoxide dismutases in terms of their three-dimensional structure and specific interactions with analogs of superoxide. The specific goal is the completion of the refined crystal structure, to at least 1.9 angstroms resolution, of the iron superoxide dismutase from Ps. ovalis. Large single crystals of the enzyme have been grown and an electron density map at 2.9 angstroms resolution has been obtained by multiple isomorphous replacement methods. The map appears interpretable in detail. We will extend the resolution, fit the amino acid sequence, refine the structure by restrained least squares and molecular averaging, and examine the structures of enzyme-inhibitor complexes. Superoxide dismutase is important in a number of health-related areas, including resistance to radiation toxicity, and we hope knowledge of its structure may facilitate clinical uses of the enzyme or analogs of it. The enzyme is also extremely useful in food preservation.